1. Field of the Invention
The present invention relates to the manufacturing in monolithic form of medium power bidirectional switches.
2. Discussion of the Related Art
The most current static bidirectional switches are triacs. A triac corresponds to the antiparallel connection of two thyristors. It can thus be directly connected in an A.C. network, for example, the mains. The gate of a conventional triac corresponds to the cathode gate of one at least of the two thyristors forming it and is referenced to the electrode located on the front surface side of this triac, that is, the surface including the gate terminal. As a result, the other surface or rear surface of the triac, which is currently connected to a radiator is at the high voltage, which poses isolation problems.
Bidirectional switches of the type described in U.S. Pat. No. 6,034,381 (B3073), the triggering of which is ensured by application of a voltage between a control electrode located on the front surface of the component and a main electrode located on the opposite surface of the component will more specifically be considered hereafter. U.S. Pat. No. 6,034,381 is hereby incorporated by reference.
FIG. 1 shows an equivalent electric diagram of such a bidirectional switch. A control electrode G of the bidirectional switch is connected to the emitter of a bipolar transistor T having its collector connected to the anode gates of first and second thyristors Th1 and Th2 arranged in antiparallel between two terminals A1 and A2. Terminal A1 corresponds to the anode of thyristor Th1 and to the cathode of thyristor Th2. Terminal A1 is also connected to the base of transistor T. Terminal A2 corresponds to the anode of thyristor Th2 and to the cathode of thyristor Th1.
FIG. 2 is a simplified cross-section view of an example of monolithic forming of the bidirectional switch described in relation with FIG. 1. Transistor T is formed in the left-hand portion of the drawing, thyristor Th1 is formed at the center, and thyristor Th2 is formed to the right.
The structure of FIG. 2 is formed from a lightly-doped N-type semiconductor substrate 1. The anode of thyristor Th1 corresponds to a P-type layer 2, which is formed on the rear surface side of substrate 1. Its cathode corresponds to an N-type region 3 formed on the front surface side in a P-type well 4. The anode of thyristor Th2 corresponds to a P-type well 5 formed on the front surface side and its cathode corresponds to an N-type region 6 formed on the rear surface side in layer 2. The structure periphery is formed of a heavily-doped P-type region 7 extending from the front surface to P-type layer 2. Conventionally, region 7 is obtained by drive-in from the two substrate surfaces. The rear surface is coated with a metallization M1 corresponding to first terminal A1 of the bidirectional switch. The upper surfaces of regions 3 and 5 are coated with a second metallization M2 corresponding to the second terminal A2 of the bidirectional switch. An N-type region 8 is formed, on the front surface side, in a P-type well 9 in contact with peripheral region 7. The surface of region 8 forms one piece with a metallization M3 connected to control terminal G of the bidirectional switch. A metallization M4 may be formed on the upper surface of peripheral region 7. Metallization M4 is not connected to an external terminal. As an alternative, well 9 may be separated from peripheral region 7 and electrically connected thereto via metallization M4.
The operation of the bidirectional switch is the following.
When terminal A2 is negative with respect to terminal A1, thyristor Th1 is likely to be on. If a sufficiently negative voltage with respect to metallization M1 is applied on gate G, the base-emitter junction of transistor T is forward biased and this transistor turns on. A vertical current ic shown in dotted lines in FIG. 2 thus flows from metallization M1, through the forward junction between layer 2 and substrate 1, then into regions 1, 9, and 8 corresponding to transistor T. There thus is a generation of carriers at the level of the junction between substrate 1 and well 9 close to the junction between substrate 1 and well 4, and thyristor Th1 is turned on. It can also be considered that the triggering of an auxiliary vertical NPNP thyristor including regions 8-9-1-2, region 9 of which forms the cathode gate region, has been caused.
Similarly, in the case where terminal A2 is positive with respect to terminal A1, the applying of a negative voltage on terminal G turns transistor T on. The carriers present in the vicinity of the junction between substrate 1 and layer 2 turn thyristor Th2 on, as will be better understood by referring to the simplified top view of FIG. 4 in which it can be seen that the region corresponding to transistor T is close to a portion of each of thyristors Th1 and Th2.
Practice reveals that this type of bidirectional switch has a non-optimal control sensitivity, that is, especially, the current required to trigger thyristor Th1 is of several hundreds of milliamperes.
The applicant has provided in unpublished French patent application 99/10412 (B4341) of Aug. 9, 1999, which is incorporated herein by reference, another embodiment in monolithic form of a bidirectional switch of the above-mentioned type, in which thryistor Th1 has a greater control sensitivity.
FIG. 3 is a simplified cross-section view of an embodiment of such a monolithic bidirectional switch. The structure of the various areas formed in semiconductor substrate 1 is identical to that illustrated in FIG. 2. The difference between the two drawings is that a region 10 having an isolation function, substantially under the above-mentioned auxiliary vertical thyristor, is provided on the rear surface side, between layer 2 and metallization M1. This also appears from FIG. 4 in which the contour of region 10 is designated by a dotted line in the bottom left-hand portion of the drawing. Layer 6, not shown in FIG. 4, occupies the entire lower surface except for the area located under P-type well 4 and the surface occupied by region 10.
Region 10 is formed of a semiconductor N-type doped material, preferably silicon oxide (SiO2).
The operation of the bidirectional switch remains substantially similar to what has been described in relation with FIG. 2. However, base current ib of transistor T, running from metallization M1 to region 8, is now deviated by the presence of region 10, according to path ib of FIG. 3.
The main current of the auxiliary vertical thyristor is also deviated, as shown by arrows ic. It can be seen that by modifying the dimensions of region 10, the passing of current ic is favored in the vicinity of the areas where it is most efficient to turn thyristor Th1 on, that is, close to the limit of well 4.
Tests performed by the applicant have shown that the triggering current of thyristor Th1 is minimized when region 10 extends to face P-type well 4 in which N-type region 3 forming the cathode of thyristor Th1 is formed.
The thickness of region 10 must be sufficiently small to initially enable the starting of transistor T by the conduction of current ib from layer 2 to region 8 via peripheral region 7. Indeed, if region 10 is too thick, the remaining thickness of layer 2 between region 10 and substrate 1 causes the existence of too high a resistance that opposes to the flowing of base current ib.
In practice, the thickness of region 10 will be smaller than that of layer 6. Indeed, layer 6 forms the cathode of thyristor Th2 and its thickness is determined by the characteristics, especially relating to the turn-on current, of this sole thyristor. The thickness of layer 6 will for example be on the order of 10 to 15 xcexcm, while the thickness of region 10 will be as small as possible.
An object of the present invention is to improve structures of the previously described type and especially to reduce their surface area for an equal power.
To achieve this and other objects, the present invention provides a monolithic bidirectional switch formed in a semiconductor substrate of a first conductivity type having a front surface and a rear surface, including a first main vertical thyristor, the rear surface layer of which is of the second conductivity type, a second main vertical thyristor, the rear surface layer of which is of the first conductivity type, a peripheral region of the second conductivity type extending from the front surface to the rear surface, a first metallization covering the rear surface, a second metallization on the front surface side connecting the front surface layers of the first and second thyristors, and a gate region of the first conductivity type in a portion of the upper surface of said peripheral region.
According to an embodiment of the present invention, the gate region is formed in a more heavily-doped portion of the upper surface of the peripheral region.
According to an embodiment of the present invention, the switch includes an additional region having a function of isolation on the rear surface side between the peripheral region and the first metallization, this additional region being interrupted under the areas corresponding to the first and second vertical thyristors.
According to an embodiment of the present invention, the additional region is made of a semiconductor material of the first conductivity type.
According to an embodiment of the present invention, the thickness of the additional region is smaller than that of the rear surface region of the second main vertical thyristor.
According to an embodiment of the present invention, the additional region is made of silicon oxide.
The foregoing and other objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, in which: